For more than 50 years microbiologists have warned against using antibiotics to fatten up farm animals. The practice, they argue, threatens human health by turning farms into breeding grounds of drug-resistant bacteria. Farmers responded that restricting antibiotics in livestock would devastate the industry and significantly raise costs to consumers. We now have empirical data that should resolve this debate. Since 1995 Denmark has enforced progressively tighter rules on the use of antibiotics in the raising of pigs, poultry and other livestock. In the process, it has shown that it is possible to protect human health without hurting farmers.

Farmers in many countries use antibiotics in two key ways: (1) at full strength to treat animals that are sick and (2) in low doses to fatten meat-producing livestock or to prevent veterinary illnesses. (It is illegal in the U.S. to sell milk for human consumption from dairy cattle treated with antibiotics.) Although even the proper use of antibiotics can inadvertently lead to the spread of drug-resistant bacteria, the habit of using a low or subtherapeutic dose is a formula for disaster: the treatment provides just enough antibiotic to kill some but not all bacteria. The germs that survive are typically those that happen to bear genetic mutations for resisting the antibiotic. They then reproduce and exchange genes with other microbial resisters. Because bacteria are found literally everywhere, resistant strains produced in animals eventually find their way into people as well. You could not design a better system for guaranteeing the spread of antibiotic resistance.